In this application we propose to develop a new system for the in vivo section of genetically modifier hematopoietic cells, based on the use of chemical inducers of dimerization (CIDs). The system used has two components: a fusion protein and a drug. The fusion protein contains a cytokine receptor signaling domain linked to a protein that provides a high affinity binding site for the CID. Addition of the CID results in dimerization of the fusion protein, thereby activating the receptor signaling domain. In recent studies we have shown that this system can be used as a pharmacologically-activatable "cell growth switch" for genetically modifier primary murine bone marrow cells. T of this application is to carry these in vitro observations into the in vivo setting. The specific aims of this proposal are: 1) To test whether CID-mediated activation of mpl allows for the in vivo selection of transduced murine stem and progenitor cells; 2) To identify and eliminate mpl maturational signaling domains to produce a derivative that is capable of proliferative signaling but incapable of maturational signaling; 3) To test whether CID-mediated activation of mpl allows for the selection and expansion of transduced human CD34+ cells in vitro and in vivo; 4) To test in vivo section in a large animal model using CIDs; 5) To test vectors that contain both a CID-selectable gene and a gamma globin gene for studies of selection in normal mice and in a mouse model of sickle cell anemia.